Photocatalysts based on bismuth oxyhalide, process for their preparation and uses thereof

ABSTRACT

The invention provides a process for the preparation of bismuth oxyhalide, comprising a precipitation of bismuth oxyhalide in an acidic aqueous medium in the presence of a reducing agent. Also provided are bismuth oxyhalide compounds doped with elemental bismuth Bi(0). The use of Bi(0) doped bismuth oxyhalide as photocatalysts in water purification is also described.

This application is the U.S. national phase of Intentional ApplicationNo. PCT/IL2014/050702 filed 4 Aug. 2014, which designated the U.S. andclaims the benefit of U.S. Provisional Application Nos. 61/862,101 filed5 Aug. 2013, and 62/007,946 filed 5 Jun. 2014, the entire contents ofeach of which are hereby incorporated by reference.

Compounds exhibiting photocatalytic activity are capable of acceleratingoxidation reactions in response to light irradiation and are hencepotentially useful in decomposing organic contaminants present in water.The TiO₂ powder manufactured by Degussa Corporation under the name P-25is an example of a commercially available photocatalyst.

Bismuth oxyhalides of the formula BiOHal, wherein Hal indicates halogenatom, are known in the art for use in the photocatalysis of oxidationreactions of organic matter under light irradiation. For example, Zhanget al. [J. Phys. Chem. C 112, p. 747-753 (2008)] reported thehydrothermal synthesis of BiOHal powders, by mixing together bismuthnitrate and an alkali halide salt (e.g., KCl, NaBr or KI) in ethyleneglycol as a solvent. The mixture was then subjected to hydrothermalprocedure in an autoclave at 160° C. for 12 hours. The crystallineproduct collected was in the form of microspheres consisting ofnanoplates.

Co-assigned WO 2012/066545 describes the preparation andcharacterization of mixed bismuth oxyhalide compounds of the formulaBiOCl_(y)Br_(1-y), which were obtained in the form of fairly sphericalparticles with an organized flower-like microstructure consisting ofindividual thin plates arranged radially in a petal-like manner. Themixed bismuth oxyhalide compounds were prepared by mixing a bismuth saltand quaternary ammonium chloride and bromide salts in an acidicenvironment.

Zhang et al. [Applied Catalysts B: Environmental, Vol. 90 Issue 3-4, p.458-462 (2009)] reported the synthesis of magnetic photocatalyst,Fe₃O₄/BiOCl nanocomposite, where the Fe₃O₄ nanocrystals are inlaid inthe BiOCl matrix flake.

Yu et al. [Journal of Materials Chemistry A, 2, 1677-1681 (2014)]reported the synthesis of BiOCl by mixing bismuth nitrate andcetyltrimethylammonium bromide (CTAB) in ethanol. A slurry was formed,following which hydrochloric acid was added dropwise. The mixture wassubjected to hydrothermal treatment for three hours at 180° C.

We have found that the addition of a reducing agent to an acidic aqueousreaction mixture comprising bismuth ions and halide salts, especiallyquaternary ammonium chloride and bromide salts, results in the reductionof some Bi³⁺ ions to afford Bi⁽⁰⁾ doped-bismuth oxyhalide catalysts withenhanced catalytic activity. The experimental results reported belowindicate that under light irradiation, the compounds of the inventionare highly effective in purifying water contaminated with organiccompounds. For example, an organic contaminant such as chlorobenzene isfully oxidized to carbon dioxide in the presence of the compound of theinvention following short exposure period to light irradiation.

Accordingly, one aspect of the invention is a process comprising aprecipitation of bismuth oxyhalide in an acidic aqueous medium in thepresence of a reducing agent. More specifically, the process comprisescombining in an acidic aqueous medium at least one bismuth salt, atleast one halide source and a reducing agent, and isolating a bismuthoxyhalide precipitate. It should be noted that by the term “aqueousmedium” is not meant that the reaction medium in question consistssolely of water. Water-miscible organic solvents may be present in thereaction mixture. For example, an organic acid such as glacial aceticacid is preferably used for generating the acidity and ethanol could beadded to the reaction mixture to act as a defoamer, as explained below.Water preferably constitutes at least 30% (v/v) of the total volume ofthe liquid medium where the reaction takes place, e.g., between onethird and two thirds of the total volume.

Optionally, a secondary metal ion M^(p+), wherein p is an integer equalto or greater than 1, and preferably equal to or greater than 2, e.g.,trivalent or quadrivalent metal ion, may be added to the reactionmixture, for example, Fe³⁺.

The process set forth above is preferably carried out by charging areaction vessel with water and an acid and dissolving the bismuth saltin the acidic environment. The resultant solution is then combined witha source of halide ion to form a reaction mixture, followed by theaddition of a reducing agent to said reaction mixture. The pH of thereaction mixture is preferably less than 4, and even more preferablyless than 3.5, e.g., from 2.5 to 3, and more specifically around 3.

A bismuth salt suitable for use in the process of the invention is abismuth compound which decomposes under acidic environment to releasebismuth ions. To this end, bismuth compounds, such as bismuth nitrate(Bi(NO₃)₃.5H₂O) or bismuth oxide (Bi₂O₃) are suitable for use, withbismuth nitrate being especially preferred.

The bismuth salt is dissolved in an acidic medium which is preferablygenerated by means of an organic acid or an aqueous solution of anorganic acid such as glacial acetic acid or formic acid. The dissolutionof the bismuth salt can be easily accomplished at room temperature(e.g., between 20° C. and 30° C.) under stirring.

The halide source(s), for example, chloride or bromide or both ispreferably selected from the group consisting of quaternary ammoniumchloride and bromide salts. Preferred salts are represented by theformulas N⁻R₁R₂R₃R₄Cl⁻ and N⁺R—R₂R₃R₄Br⁻, wherein R₁, R₂, R₃ and R₄ arealkyl groups, which may be the same or different. For example, R₁, R₂and R₃ are short chain alkyl groups (e.g., methyl groups) and R₄ is along straight or branched alkyl chain, preferably a straight chainconsisting of not less than 12 carbon atoms (e.g., not less than 16carbon atoms). For example, halide sources which can be suitably usedare selected from the group consisting of cetyltrimethylammonium bromide(abbreviated CTAB), cetyltrimethylammonium chloride (abbreviated CTAC),tetrabutylammonium chloride (abbreviated TBAC) and tetrabutylammoniumbromide (abbreviated TBAB). The cationic surfactants described above(e.g., CTAB and CTAC) appear to function as Structure DirectingAgents—SDAs, affecting the morphological structure of the resultantcompounds, as discussed below. However, it should be noted that metalhalide salts such as alkali halides (e.g., NaCl, KBr) can also be usedas halide sources.

The presence of a secondary metal ion M^(p+), such as Fe³⁺, is notmandatory. Ferric compounds which can be used include ferric nitrateFe(NO₃)₃.6H₂O and salts of the formula AFe(SO₄)₂.12H₂O wherein Aindicates a unipositive cation such as alkali or ammonium.

When two organic halide salt(s) are used to form a mixed bismuthoxyhalide, they can be added either simultaneously or in succession tothe bismuth-containing acidic solution. The halide salts can be employedin a solid form or more preferably, in the form of separate or combinedaqueous or alcoholic solutions. For example, when the reaction iscarried out also in the presence of Fe³⁺ source, one convenient order ofaddition involves the premixing of organic halide(s) and ferric salts inwater, followed by the addition of the acidic bismuth-containingsolution to form a reaction mixture.

The concentration of the bismuth salt in the acidic reaction mixture ispreferably from 0.02 M up to saturation limit, for example from 0.05 to0.5 M. When a mixture of water and a liquid organic acid is used, thenthe volumetric ratio between the aqueous and organic components ispreferably from 2:1 to 1:2, e.g., around 1:1. The chloride or bromidesalts are used in stoichiometric amounts relative to the bismuth source,or in a slight molar excess. If ferric ions are added, then the molarratio between the bismuth and ferric ions is preferably in the rangefrom 200:1 to 100:5, for example, around 100:1.

As already mentioned above, the process of the invention relates also tothe precipitation of a mixed bismuth oxyhalide, e.g., chloride-bromidebismuth oxyhalides. To this end, the precipitation reaction takes placein the presence of different halides, e.g., chloride and bromidesources, with the molar ratio chloride/bromide being adjusted to givethe desired BiOCl_(y)Br_(1-y) compound, where y is greater than 0.5,e.g., from 0.5 to 0.95, e.g., from 0.6 to 0.95, preferably from 0.7 to0.95. Preferably, the amounts of the chloride and bromide salts areadjusted to form the mixed BiOCl_(y)Br_(1-y) compound in which the ratioy/1-y is not less than 2:1, and preferably from 3:1 to 8:1, inclusive.

On combining a bismuth salt and one or more quaternary ammonium halidesalts(s) under acidic conditions, bismuth oxyhalide begins tocrystallize almost instantaneously to give spherical particles withflower-like morphology, composed of ‘leaves’ that are interconnected toform cells or channels which open onto the external surface of thespheres. Without wishing to be bound by theory, it is believed that inthe presence of a reducing agent, some Bi³⁺ ions undergo a reductionreaction which starts almost concurrently with the precipitationreaction. This reduction reaction presumably takes place within the opencells and channels of the bismuth oxyhalide particles, incorporatingbismuth metal as a dopant into the bismuth oxyhalide particles, therebyaffecting the structure and properties of the particles.

The bismuth ion reduction is preferably accomplished in the presence ofan inorganic hydride as a reductant, such as metal borohydride. Thereduction may proceed to the formation of metal bismuth according to thefollowing reaction:4Bi³⁺ _((aq))+3BH₄ ⁻ _((aq))+9H₂O→4Bi⁰ _((s))+3H₂BO₃ ⁻ _((aq))+12H⁺_((aq))+6H_(2(g))

The borohydride, e.g., sodium borohydride, is introduced into thereaction vessel in a solid form and the reaction mixture is vigorouslystirred. Alternatively, borohydride solution is prepared in advance andfed to the reaction mixture. In general, the amount of borohydride isadjusted to achieve Bi⁰ doping level in the photocatalyst of not morethan 7 molar %, e.g., from 0.1 to about 7 molar %, more specificallyfrom 0.1 to 5 molar % (e.g., 0.1 to 3 molar %), relative to the totalamount of the bismuth. The borohydride may be applied in a small excessover the 3:4 molar ratio required according to the chemical equation setforth above to accelerate the reduction reaction and achieve the desireddoping level.

The metal reduction by means of borohydride is simple and safe. However,the reduction reaction is prone to foam formation, due to the presenceof quaternary ammonium halide surfactants and sodium borohydride in anaqueous environment leading to evolution of elemental hydrogen. Theaddition of a small amount of a defoamer, e.g., a water miscible organicco-solvent such as ethanol concurrently with the feeding of thereductant, enables the reaction to proceed in a mild manner, with theethanol functioning as an anti-foaming agent, allowing the homogeneousmixing of reagents. It should be noted that bismuth reduction can alsobe carried out using other reducing agents, such as LiAlH₄ (lithiumaluminum hydride).

Following the addition of the reducing agent, the reduction reaction iscompleted within a very short period of time, e.g., not more than a fewminutes. However, the reaction mixture may be kept under stirring atroom temperature or slightly above room temperature for not less than 10minutes. The solid is then separated from the liquid phase by means ofconventional methods, such as filtration or decantation, washedthoroughly (e.g., with ethanol and water), and dried in air or in avacuum oven. It is noted the process of the invention is devoid of astep of hydrothermal treatment of the reaction mixture in an autoclave.

The filtrate produced consists of a mixture of water, an organic acid(e.g., acetic acid), optionally an organic co-solvent (e.g., ethanol),quaternary ammonium cation of the formula N⁻R₁R₂R₃R₄ as defined aboveand various counter ions of the reactants employed in the process. Thefiltrate can serve as a reaction medium for a successive productionbatch. The filtrate is easily regenerated; fresh amounts of thosereactants which were consumed by the reaction are added to the filtrate,save for the quaternary ammonium salt surfactant whose activity asstructure-directing agent can be restored in a satisfactory manner bymeans of an addition of a water soluble halide salt. Thus, the bismuthsalt, a reducing agent and optionally an iron salt are fed to thefiltrate together with water-soluble halide salt (e.g., alkali halidesuch as sodium chloride) and the regenerated filtrate is supplied to thenext production batch for use as a reaction medium. The products whichare formed in a fresh or a recycled reaction medium are equally good inrespect of their photocatalytic activity. The process of the inventionis hence readily applicable for large scale production either in a batchor continuous mode of operation.

Accordingly, another aspect of the invention is a process comprisingcombining one or more bismuth salts and at least one quaternary ammoniumhalide salt in the presence of a reducing agent (and optionally, ifdesired, M^(p+) ion, e.g., trivalent metal such as ferric ion), in anacidic aqueous (or aqueous organic) reaction medium, separating aprecipitate from the liquid reaction medium, collecting the liquidphase, adding water soluble halide salt to said liquid phase andrecycling the same as an acidic aqueous reaction medium for bismuthoxyhalide precipitation.

Bismuth oxyhalide obtainable by the foregoing process forms anotheraspect of the invention. Due to the reduction of bismuth ions and theformation of elemental bismuth as a dopant, the novel compounds areidentified as Bi⁽⁰⁾-doped bismuth oxyhalide, which are especiallyselected from the group consisting of Bi⁽⁰⁾doped-BiOCl, Bi⁽⁰⁾doped-BiOBrand Bi⁽⁰⁾doped-BiOCl_(y)Br_(1-y) wherein y is in the range from 0.5 to0.95, preferably from 0.6 to 0.95 (e.g., 0.7 to 0.95). Photoelectronspectroscopy can be used for the analysis of the composition of thecatalyst and determination of the chemical state of bismuth present inthe catalyst, e.g., on the catalyst surface. The binding energies of theBi metal 4f band are ˜157 eV and ˜162 eV. For bismuth compounds, such asBi₂O₃, the peaks located at ˜159 eV and ˜164 eV are assigned to Bi 4f7/2 and Bi 4f 5/2, respectively. X-ray photoelectron spectrum of asample of a compound of the invention displays peaks at binding energiesof 157±1 eV and 162±1 eV, assigned to the Bi (_(metal)) 4f (7/2, 5/2)photoelectrons, respectively [in addition to the peaks assigned to Bi(_(BiOCl, BiOBr, and BiOClyBr1-y)) 4f (7/2, 5/2)]. Thus, the inventionprovides a compound selected from the group consisting ofBi⁽⁰⁾doped-BiOCl, Bi⁽⁰⁾doped-BiOBr and Bi⁽⁰⁾doped-BiOCl_(y)Br_(1-y)characterized in that its X-ray photoelectron emission spectrum exhibitsa peak at 157±1 eV assigned to metallic bismuth. The molar concentrationof the Bi⁽⁰⁾ dopant in the compounds of the invention is preferably notmore than 7 molar %, e.g., from 0.1 to 7.0% molar, more specificallyfrom 0.1 to 5.0% molar, e. g., from 0.1 to 3.0 molar %, more preferablyfrom 0.5 to 3% (for example, from 1.0 to 3.0 molar %). The molarpercentage of the dopant is calculated relative to the total amounts ofthe trivalent and zerovalent bismuth.

The bismuth oxyhalide of the invention are crystalline, as demonstratedby their X-ray powder diffraction patterns. For example, bismuthoxychloride of the invention exhibit characteristic peaks at 12.022θ±0.05 and one or more peaks at 26.01, 32.25, 40.82 and 58.73 2θ (±0.052θ). Bismuth oxybromide of the invention exhibits characteristic peaksat 11.0 2θ±0.05 and one or more peaks at 31.78, 32.31, 39.26, 46.31,57.23, 67.53 2θ±0.05. The mixed BiOCl_(y)Br_(1-y) compounds of theinvention exhibit X-ray powder diffraction pattern having acharacteristic peak in the range from 11.0 to 12.2 2θ (±0.05 2θ), whichpeak is indicative of the Cl:Br ratio. In other words, the exactposition of the indicative peak within the 11.0-12.2 2θ interval dependsessentially linearly on the Cl:Br ratio, as predicted by the Vegardrule. The chemical composition of the compound belonging to the familyBiOCl_(y)Br_(1-y) wherein y is as defined above can be determined usingEDS analysis. The composition of the BiOCl_(y)Br_(1-y) compound can bealso determined using XRD data and Vegard's law.

Images recorded with scanning electron microscopy indicate that thebismuth oxyhalide particles are largely spherical, in the form ofmicrospheres exhibiting flower-like surface morphology. By the term“flower-like surface morphology” is meant that the spherical particlesare characterized by the presence of individual thin sheets or platesarranged radially like petals, wherein two or more adjacent individualthin sheets are interconnected to form cells or channels which open ontothe external surface of said spheres.

Particle size measured with Malvern Instruments—Mastersizer 2000particle size analyzer shows that the average diameter of the sphericalparticles is from 2 to 5 microns, more specifically from 3 to 4 microns.

The preferred compounds provided by the present invention have a surfacearea of not less 27 m²/g, more preferably not less than 30 m²/g, e.g.,from 30 to 35 m²/g, as determined by BET (the nitrogen adsorptiontechnique).

The compounds of the present invention can be used in the light-inducedcatalysis of oxidation reactions of chemical pollutants. The compoundsof the invention have been found to exhibit high photocatalytic activityin decomposing organic contaminants present in water under UV-Vis andvisible light irradiation. Specifically, the compounds of the inventionmay be used in purifying water contaminated by organic substances suchas dyes and aromatic or heteroaromatic compounds which may besubstituted by various chemical groups such halogen, hydroxyl,carboxylic acid, amine and keto functionalities. As illustrated in theExamples below, the compounds of the invention are useful in advancingoxidation and degradation of organic compounds including aromaticcompounds such as phenol and halogen-substituted benzene, e.g.,chlorobenzene, at very fast rates. The bismuth oxyhalide of theinvention are capable of achieving mineralization of various organiccontaminants, and even total mineralization, i.e., an essentially fulloxidation of the pollutant to generate carbon dioxide.

The combination of bismuth oxyhalide of the invention and hydrogenperoxide demonstrates high efficacy in oxidizing organic pollutants. Inthe absence of bismuth oxyhalide, hydrogen peroxide alone does notappear to be able to advance the oxidation and decomposition of theorganic contaminants. However, when hydrogen peroxide is added to acontaminated aqueous medium in which the bismuth oxyhalide is activeunder light irradiation, then the rate of decomposition of the organiccontaminants is increased. When combined with the bismuth oxyhalide ofthe invention, hydrogen peroxide may be added to the aqueous system inneed of purification at a concentration as low as 10 ppm. Concentrationabove 10 ppm are generally applied, e.g., of not less 0.01M, andspecifically between 0.01 and 0.03M. As shown below, a mixture ofbismuth oxyhalide and hydrogen peroxide appears to exhibit usefulsynergy in water decontamination.

Accordingly, in another aspect, the invention provides a method for thepurification of water, comprising adding the photocatalyst of theinvention as identified above to water contaminated with organiccompound(s) and light irradiating the photocatalyst (e.g., with UV-Vislight or visible light), optionally in the presence of hydrogenperoxide. Preferably, the organic pollutants undergo mineralization andmore preferably total mineralization.

For example, the purification method of the present invention may beconducted by feeding the contaminated water to be treated into asuitable reactor, e.g., a plug flow reactor loaded with the catalyst(e.g., in a granular form), and irradiating said reactor, preferably atroom temperature. Contaminated water is circulated through theirradiated packed reactor at a selected flow rate to secure a desiredlevel of purification. Catalytically effective concentration of thebismuth oxyhalide may vary from 100 ppm to 1000 ppm, e.g., an amount of250 to 700 ppm of the catalyst is added to an aqueous system which maytypically contain contaminants at a concentration of up to 1000 ppm.

Useful UV-Vis and visible light sources include xenon arc lamps, halogenlamps or lasers. Solar irradiation is also effective. In general, theirradiation period depends on the identity of the organic contaminant tobe destroyed, its concentration in the aqueous medium, the catalystemployed and the loading of the catalyst in the reactor. The irradiationperiod is not less than 3-5 minutes, e.g., between about 10 minutes andseveral hours, and the progress of the decomposition of the targetedcontaminants can be monitored using conventional techniques, such asspectroscopic methods in order to determine that a characteristicabsorption peak has reduced in intensity or completely vanished, or bymeasuring Chemical Oxygen Demand (COD) or Total Organic Carbon (TOC) ofthe water under treatment.

Bismuth oxyhalide can also be applied in the form of a thin film ontothe surface of a suitable substrate, e.g., made of glass or aluminum.Co-assigned WO 2012/066545 discloses that bismuth oxyhalide compoundscan be embedded in an adhesive matrix (e.g., siloxane-based matrix)which can be affixed to a surface of a substrate in the form of a thinfilm demonstrating photocatalytic activity in response to sun lightirradiation. WO 2012/066545 specifically exemplifies the coating of aglass substrate with a dispersion of the photocatalyst in siloxane-basedsystem.

We have now found a versatile method for creating bismuthoxychloride-containing thin films onto the surface of a substrate,including a surface which exhibits slight roughness. The method consistsof preparing two separate solutions: a first solution which contains thecoating-forming materials and either bismuth or halide source (the“coating solution”), and a second solution which contains the counterion (either halide or Bi³⁺, respectively). The coating solution isapplied onto the surface of the substrate, for example by a dip coatingtechnique whereby the substrate is immersed in the coating solution andpulled up to deposit the coating layer, followed by spraying the secondsolution onto the coating layer, to allow the instantaneousprecipitation of the bismuth oxyhalide, thereby creating bismuthoxychloride-containing thin film.

The coating solution is preferably prepared by first dissolving thefilm-forming material (e.g., a siloxane precursor such as tetraethylorthosilicate) in an acidic aqueous solution, in which a water-miscibleorganic co-solvent is also present (e.g., ethanol). For this purpose, amineral acid such as nitric acid can be used. Then, one or moreauxiliary organic compounds which are decomposable under calcinationconditions are added to the siloxane solution, to improve the propertiesof the film to be produced. For example, in order to assure that thecatalyst particles are uniformly dispersed in the film, long chainpolymers, e.g., poloxamer triblock copolymers (such as Pluronic P123)are added to the coating solution. Pore forming agents such as polyvinylalcohol (PVA) may be added to the coating solution, such that uponremoval during the final calcination step, pores are formed in thesiloxane matrix. Finally, a solution of a bismuth salt is added to themixture, which is vigorously stirred until a uniform coating solution isobtained.

The second solution is readily prepared by dissolving the halide source,e.g., the quaternary ammonium halide, in water-alcohol solution andadding the solution to a spraying device.

Finally, a thoroughly cleaned surface of a suitable substrate (e.g., atransparent glass) is dip coated with the coating solution and thehalide-containing solution is gently sprayed onto the coating. Followingcalcination (up to 400° C. under slow heating rate), a thin film (with athickness varying from 1 to 100 μm) is formed, affixed to the surface ofthe substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a characteristic X-ray powder diffraction pattern of the BiOClcompound of the invention.

FIG. 2 is an image recorded with a scanning electron microscope showingBiOCl particles with flower-like morphology.

FIG. 3 shows the X-ray photoelectron emission spectrum of theBi⁽⁰⁾doped-BiOCl of the invention.

FIG. 4 is a characteristic X-ray powder diffraction pattern of the BiOClcompound obtained by a process of the invention involving a filtraterecycling.

FIG. 5 is an image recorded with a scanning electron microscope showingBiOCl particles with flower-like morphology obtained by a process of theinvention involving a filtrate recycling.

FIG. 6 is a characteristic X-ray powder diffraction pattern of the BiOBrcompound of the invention.

FIG. 7 is an image recorded with a scanning electron microscope showingBiOBr particles with flower-like morphology.

FIG. 8 is a characteristic X-ray powder diffraction pattern of a mixedbismuth oxyhalide BiOCl_(0.875)Br_(0.125) compound of the invention.

FIG. 9 shows the X-ray photoelectron emission spectrum of theBi⁽⁰⁾doped-BiOCl_(0.875)Br_(0.125) of the invention.

FIG. 10 shows the X-ray photoelectron emission spectrum of theBi⁽⁰⁾doped-BiOCl_(0.875)Br_(0.125) of the invention.

FIG. 11 shows the X-ray photoelectron emission spectrum of theBi⁽⁰⁾doped-BiOBr of the invention.

FIG. 12 shows the X-ray photoelectron emission spectrum of theBi⁽⁰⁾doped-BiOCl_(0.67)Br_(0.33) of the invention.

FIG. 13 presents UV spectra demonstrating the progress of chlorobenzenedegradation in an aqueous solution in the presence of the compound ofthe invention activated with light irradiation.

FIG. 14 presents UV spectra demonstrating the progress of chlorobenzenedegradation in aqueous solution in the presence of the compound of theinvention activated with light irradiation.

FIG. 15 presents UV spectra of chlorobenzene in aqueous solution in thepresence of hydrogen peroxide.

FIG. 16 presents UV spectra demonstrating the progress of chlorobenzenedegradation in aqueous solution in the presence of both the compound ofthe invention activated with light irradiation and hydrogen peroxide.

FIG. 17 presents UV spectra demonstrating the progress of phenoldegradation in aqueous solution in the presence of the compound of theinvention activated with light irradiation.

FIG. 18 presents UV spectra of phenol in aqueous solution in thepresence of hydrogen peroxide.

FIG. 19 presents UV spectra demonstrating the progress of phenoldegradation in water in the presence of the compound of the inventionunder light irradiation and hydrogen peroxide.

FIG. 20 presents UV spectra demonstrating the progress of phenoldegradation in water in the presence of both the compound of theinvention activated with light irradiation and hydrogen peroxide.

FIG. 21 presents C/C₀ plot versus time, illustrating the degradation oftoluene in an aqueous medium in the presence of the compound of theinvention under light irradiation.

FIG. 22 is C/C₀ plot versus time, illustrating the degradation of methylblue (MB) in water in the presence of a compound of the invention underlight irradiation.

FIG. 23 presents C/C₀ plot versus time, illustrating the degradation ofcarbamazepine in an aqueous medium in the presence of the compound ofthe invention under light irradiation.

EXAMPLES

Methods

XRD measurements were performed on D8 Advance diffractometer (BrukerAXS, Karlsruhe, Germany) with a goniometer radius 217.5 mm, Gabel Mirrorparallel-beam optics, 2° Sollers slits and 0.2 mm receiving slit. Lowbackground quartz sample holder was carefully filled with the powdersamples. XRD patterns from 5° to 85° 2θ were recorded at roomtemperature using CuKα radiation (λ=0.15418 nm) with the followingmeasurement conditions: tube voltage of 40 kV, tube current of 40 mA,step scan mode with a step size 0.02° 2θ and counting time of is perstep for preliminary study and 12 s per step for structural refinement.The instrumental broadening was determined using LaB₆ powder(NIST-660a).

Morphological observations and identification of chemical compositionwere performed with the HRSEM-High Resolution Scanning ElectronMicroscope-Sirion (equipped with EDS LN2 detector, Oxford instruments,UK).

XPS analysis was conducted using XPS Kratos AXIs Ultra (KratosAnalytical Ltd., UK) high resolution photoelectron spectroscopyinstrument.

UV spectroscopy analysis was carried out by means of UV-visspectrophotometer (Varian EL-03097225).

Chemical Oxygen Demand (COD) was measured using COD meter-DIN38404-C3standard.

Total organic carbon (TOC) was measured using PF-11 photometer.

Example 1 Preparation of Bismuth Oxychloride in the Presence of aReducing Agent and Ferric Ions

Deionized water (40 ml), glacial acetic acid (40 ml) and bismuth nitratepentahydrate Bi(NO₃)₃.5H₂O (9.7 g) are placed in 250 ml flask andstirred at room temperature for fifteen minutes until a clear solutionis formed. The solution is added to a second flask which was previouslycharged with CTAC (25.6 g of 25 wt % CTAC aqueous solution) and ammoniumiron(III) sulfate NH₄Fe(SO₄)₂ (0.096 g). Sodium borohydride (0.01 g) andethanol (10 ml) are also added to the reaction mixture, which is stirredfor additional 60 minutes at about 30° C.

The precipitate thus formed is separated from the liquid phase byfiltration, washed five times with ethanol (5×50 ml) and then five timeswith water (5×200 ml). The off-white solid product is then dried (3hours in air). The weight of the dried solid collected is ˜7 g.

The X-ray powder diffraction pattern of the resultant bismuthoxychloride is presented in FIG. 1. The product exhibits X-ray powderdiffraction pattern having characteristic peaks at 12.02, 26.01, 32.25,40.82, 58.73 2θ (±0.05 20). The product is characterized by averageparticle size of 3 μm and surface area of 31 m²/g. FIG. 2 presents SEMimage of the particles showing their flower-like morphology. Theparticles are relatively uniform in size, e.g., a representative singleparticle size is about 3 μm.

XPS was used for the analysis of the composition of the solid. FIG. 3shows the X-ray photoelectron emission spectrum of the sample. The peaksat binding energies of ˜156.9 eV and 162.2 eV are assigned to theBi(_(metal)) 4f (7/2, 5/2) photoelectrons, respectively. The compound isidentified as Bi⁽⁰⁾doped-BiOCl.

Example 2 Precipitation of Bismuth Oxychloride from a Recycled Filtratein the Presence a Reducing Agent and Ferric Ions

The filtrate obtained following the separation of the solid product inExample 1 was reused as a reaction medium in this Example. The filtratecontains acetic acid, ethanol, water and the cationic part of thesurfactant. To this filtrate were added Bi(NO₃)₃.5H₂O (9.7 g), ammoniumiron sulphate (0.096 g dissolved in 5 ml water), sodium borohydride(0.01 g), sodium chloride (1.17 g) and ethanol (10 ml). The reactionmixture was allowed to stand for 60 minutes under mixing at 30° C.

The precipitate thus formed is separated from the liquid phase byfiltration, washed five times with ethanol (5×50 ml) and then five timeswith water (5×200 ml). The off-white solid product is then dried (3hours in air). The weight of the dried solid collected is ˜7 g.

The X-ray powder diffraction pattern shown in FIG. 4 and the SEM imageof FIG. 5 are comparable to the XRPD and SEM image of FIGS. 1 and 2,respectively, demonstrating that the crystallinity and particlemorphology of bismuth oxyhalide which precipitates from a recycledfiltrate and from a fresh reaction medium (Example 1) are essentiallythe same.

Example 3 Preparation of Bismuth Oxybromide in the Presence of aReducing Agent and Ferric Ions

Deionized water (40 ml), glacial acetic acid (40 ml) and bismuth nitratepentahydrate Bi(NO₃)₃.5H₂O (9.7 g) are placed in 250 ml flask andstirred at room temperature for fifteen minutes until a clear solutionis formed. The solution is added to a second flask which was previouslycharged with CTAB solution (7.28 g dissolved in 20 ml of water) andammonium iron(III) sulfate NH₄Fe(SO₄)₂ (0.48 g). Sodium borohydride(0.04 g) and ethanol (10 ml) are added to the reaction mixture which isstirred for additional 60 minutes at about 30° C.

The precipitate thus formed is separated from the liquid phase byfiltration, washed five times with ethanol (5×50 ml) and then five timeswith water (5×200 ml). The off-white solid product is then dried (3hours in air). About 7 g of a slightly hygroscopic product werecollected, containing ˜5-10% water.

The X-ray powder diffraction pattern of the resultant bismuth oxybromideis presented in FIG. 6. The product exhibits X-ray powder diffractionpattern having characteristic peaks at 11.00, 31.78, 32.31, 39.26,46.31, 57.23, 67.53 (±0.05 2θ). The product is characterized by averageparticle size of 3 μm and surface area of 30 m²/g. FIG. 7 presents SEMimage of the particles showing their flower-like morphology. Theparticles are relatively uniform in size; a representative singleparticle size is 3 μm.

Example 4 Preparation of Mixed Halide BiOCl_(0.875)Br_(0.125) in thePresence of a Reducing Agent and Ferric Ions

Deionized water (45 ml), glacial acetic acid (50 ml) and bismuth nitrate(14.69 g) are added to a flask and are mixed at room temperature forfifteen minutes until a clear, transparent solution is formed. CTAB(1.378 g dissolved in 10 ml of water), CTAC (8.48 g in the form of 25 wt% aqueous solution) and ammonium iron (III) sulfate (146 mg dissolved in10 ml water) are added to the bismuth solution. Finally, sodiumborohydride (0.015 g) and ethanol (10 ml) are added to the reactionmixture, which is then stirred for additional 60 minutes at about 30° C.

The precipitate thus formed is separated from the liquid phase byfiltration, washed five with ethanol (5×50 ml) and then five times withwater (5×200 ml). The solid is then dried in air. The weight of thesolid collected is ˜10.5 grams.

The X-ray powder diffraction pattern of the resultant mixed bismuthoxyhalide is presented in FIG. 8. The product exhibits X-ray powderdiffraction pattern having a characteristic peak at 11.74 2θ (±0.05 2θ)and additional peaks at 32.56, 36.06, 46.70 and 49.41 2θ (±0.05 2θ). Theproduct is characterized by average particle size of 1 μm and surfacearea of 34 m²/g.

XPS was used for the analysis of the composition of the solid. FIG. 9shows the X-ray photoelectron emission spectrum of the sample. The peaksat binding energies of ˜156.9 eV and 162.2 eV are assigned to theBi(_(metal)) 4f (7/2, 5/2) photoelectrons, respectively. The product isidentified as Bi⁽⁰⁾doped-BiOCl_(0.875)Br_(0.125).

Example 5 Preparation of Bi⁽⁰⁾ Doped-Mixed HalideBiOCl_(0.875)Br_(0.125)

Deionized water (50 ml), glacial acetic acid (40 ml) and bismuth nitrate(14.69 g) are added to a flask and are mixed at room temperature forfifteen minutes until a clear, transparent solution is formed. Theso-formed solution is added to a previously prepared solution consistingof CTAC (33.92 g of 25 wt % aqueous solution) and CTAB (1.38 g).Finally, sodium borohydride (11.456 mg) and ethanol (20 ml) are added tothe reaction mixture, which is then stirred for additional 60 minutes atabout 25-30° C.

The precipitate thus formed is separated from the liquid phase byfiltration, washed five with ethanol (5×50 ml) and then five times withwater (5×200 ml). The off-white solid is then dried (3 hours in air).The weight of the solid collected is ˜9 grams.

Example 6 Preparation of Bi⁽⁰⁾ Doped-Mixed HalideBiOCl_(0.875)Br_(0.125)

The procedure of Example 5 was repeated, with a twofold increase of theamount of the reducing agent (22.913 mg of sodium borohydride is addedto the reaction mixture).

The precipitate thus formed is separated from the liquid phase byfiltration, washed five with ethanol (5×50 ml) and then five times withwater (5×200 ml). The off-white solid is then dried (3 hours in air).The weight of the solid collected is ˜9 grams.

XPS was used for the analysis of the composition of the solid. FIG. 10shows the X-ray photoelectron emission spectrum of the sample. The peaksat binding energies of ˜157.15 eV and 163.8 eV are assigned to theBi(_(metal)) 4f (7/2, 5/2) photoelectrons, respectively. The product isidentified Bi⁽⁰⁾doped-BiOCl_(0.875)Br_(0.125).

Example 7 Preparation of Bi⁽⁰⁾ Doped-BiOBr

Deionized water (50 ml), glacial acetic acid (40 ml) and bismuth nitrate(9.7 g) are added to a flask and are mixed at room temperature forfifteen minutes until a clear, transparent solution is formed. Theso-formed solution is added to a previously prepared aqueous ethanolicsolution of CTAB (1.38 g CTAB dissolved in a mixture consisting of 30 mlethanol and 10 ml deionised water). Finally, sodium borohydride (7.56mg) is added to the reaction mixture, which is then stirred foradditional 60 minutes at about 25-30° C.

The precipitate thus formed is separated from the liquid phase byfiltration, washed five with ethanol (5×50 ml) and then five times withwater (5×200 ml). The off-white solid is then dried (3 hours in air).The weight of the solid collected is ˜7 grams.

XPS was used for the analysis of the composition of the solid. FIG. 11shows the X-ray photoelectron emission spectrum of the sample. The peaksat binding energies of ˜156.8 eV and 164.9 eV are assigned to theBi(_(metal)) 4f (7/2, 5/2) photoelectrons, respectively. The product isidentified as Bi⁽⁰⁾doped-BiOBr.

Example 8 Preparation of Bi⁽⁰⁾ Doped-Mixed Halide BiOCl_(0.67)Br_(0.33)

Deionized water (50 ml), glacial acetic acid (40 ml) and bismuth nitrate(3.27 g) are added to a flask and are mixed at room temperature forfifteen minutes until a clear, transparent solution is formed. Theso-formed solution is added to a previously prepared solution consistingof CTAC (3.2 g in 25 wt % aqueous solution) and CTAB (1.82 g). Finally,sodium borohydride (5.70 mg) and ethanol (20 ml) are added to thereaction mixture, which is then stirred for additional 60 minutes atabout 25-30° C.

The precipitate thus formed is separated from the liquid phase byfiltration, washed five with ethanol (5×50 ml) and then five times withwater (5×200 ml). The off-white solid is then dried (3 hours in air).The weight of the solid collected is ˜5 grams.

XPS was used for the analysis of the composition of the solid. FIG. 12shows the X-ray photoelectron emission spectrum of the sample. The peaksat binding energies of ˜157.75 eV and 163.06 eV are assigned to theBi(_(metal)) 4f (7/2, 5/2) photoelectrons, respectively. The product isidentified as Bi⁽⁰⁾doped-BiOCl_(0.67)Br_(0.33).

Some of the photocatalysts prepared in the foregoing examples aretabulated in Table A.

TABLE A Bi⁽⁰⁾ 4f 7/2 Dopant Example Compound XPS peak level 1 Bi⁽⁰⁾doped-BiOCl 156.9 eV (FIG. 3) ~1 mole % 4, 5 Bi⁽⁰⁾doped- 156.9 eV (FIG.9) ~1 mole % BiOCl_(0.875)Br_(0.125) 6 Bi⁽⁰⁾doped- 157.1 eV (FIG. 10) ~2mole % BiOCl_(0.875)Br_(0.125) 7 Bi⁽⁰⁾ doped-BiOBr 156.8 eV (FIG. 11)~1.5 mole %   8 Bi⁽⁰⁾doped- 157.7 eV (FIG. 12) ~3 mole %BiOCl_(0.670)Br_(0.330)

Examples 9-16 Water Decontamination: Decomposition of OrganicContaminants in Aqueous Medium in the Presence of the Compound of theInvention Under Light Irradiation

Samples were prepared by adding an organic compound (eitherchlorobenzene or phenol) to 200 ml of water. The compound of Example 1was added in varying amounts to the samples and was tested for itsphotocatalytic activity on destruction of the organic compound underirradiation with Xenon lamp (300 W) located at a distance of 10 cm fromthe sample, at wavelength 250-740 nm. The compound of Example 1 was usedeither alone or in combination with hydrogen peroxide (30% aqueous H₂O₂solution). Table 1 below shows the concentration of the organiccontaminant in the sample, the amount of the compound of Example 1present in the sample and the volume of 30% hydrogen peroxide solutionadded to the sample. For the purpose of comparison, the oxidationactivity of hydrogen peroxide alone was also evaluated, i.e., in theabsence of the compound of the invention.

The decomposition of the organic compound under the conditions set forthabove was determined by periodically analyzing the tested sample bymeans of UV spectroscopy. The spectra obtained for each experiment arepresented in FIGS. 13-20, which correspond to Examples 9 to 16,respectively. The spectra show that in the presence of the photocatalystof the invention, the intensity of the characteristic UV absorbance peakassigned to the organic contaminant (for chlorobenzene ˜262 nm, phenol˜270 nm) decreases gradually with the passage of time, until the peakfinally vanishes, indicating full oxidation of the organic compound tocarbon dioxide.

TABLE 1 Organic BiOCl of Hydrogen Contaminant Example 1 Peroxide Example(concentration) (mg) (ml) Observations  9 Chlorobenzene 100 0 Fulldecomposition of (200 ppm) the contaminant after 12 minutes (FIG. 13) 10Chlorobenzene 200 0 Full decomposition of (400 ppm) the contaminantafter 20 minutes (FIG. 14) 11 Chlorobenzene 0 0.2 No decompositioncomparative (200 ppm) (FIG. 15) 12 Chlorobenzene 200 0.2 Fulldecomposition of (400 ppm) the contaminant after 16 minutes (FIG. 16) 13Phenol 100 0 Full decomposition of (50 ppm) the contaminant after 120minutes (FIG. 17) 14 Phenol 0 0.2 Transformation of phenol comparative(500 ppm) into phenol derivatives (FIG. 18) 15 Phenol 150 0.2 During thefirst three (500 ppm) hours mainly transformation into phenolderivatives is observed; then the organic compounds begin to decomposeand full decomposition is reached after 300 minutes (FIG. 19) 16 Phenol200 0.5 During the first three (1000 ppm) hours mainly transformationinto phenol derivatives is observed; then the organic compounds begin todecompose and full decomposition is reached after 540 minutes (FIG. 20)

Comparative Examples 11 and 14 illustrate that hydrogen peroxide aloneis unable to promote the oxidation of the organic compound. The resultsshown in Examples 10, 11 and 12 demonstrate that the combination of thebismuth oxyhalide and hydrogen peroxide exhibits a synergistic effect.

Example 17 Water Purification: Decomposition of Organic Contaminants inAqueous Medium in the Presence of the Compound of the Invention UnderLight Irradiation

The Bi⁽⁰⁾-doped BiOCl compound of Example 1 was tested for its abilityto purify water contaminated with chlorobenzene. The tested sampleconsisted of 200 ml aqueous solution which contains chlorobeneze (400ppm) and the compound of Example 1 (200 mg). The sample was exposed tolight irradiation as set out in previous examples, and the progressivelyreduced amount of the organic compound present in the sample wasindirectly evaluated by periodically measuring the Chemical OxygenDemand (COD). The time intervals at which the COD was measured and CODvalues are tabulated in Table 2.

TABLE 2 Irradiation time COD (min) (ppm) 0 200 4 60 8 50 20 <30 30 <30

The COD test is in line with the UV spectroscopy analysis reported inthe foregoing examples: both methods indicate that the compound of theinvention is highly effective in decontaminating water contaminated withchlorobenezene.

Examples 18-20 Water Purification: Decomposition of Organic Contaminantsin Aqueous Medium in the Presence of the Compound of the Invention UnderLight Irradiation

The Bi⁽⁰⁾doped-BiOCl_(0.875)Br_(0.125) compound of Example 4 was testedfor its ability to purify water contaminated with mixtures of organiccompounds. The tested sample consisted of 200 ml aqueous solution whichcontains the organic contaminants as tabulated in Table 3 below and thecompound of Example 4 (150 mg). The pH of the sample was approximately5. The sample was exposed to light irradiation at wavelength 385-740 nm.Light intensity was 70 mW/cm² and the lamp was located at a distance of10 cm from the sample.

The catalytic activity of the Bi⁽⁰⁾-doped BiOCl_(0.875)Br_(0.125)compound of Example 4 is evaluated by determining the time needed inorder to reduce the initial TOC value of the tested sample to a finallevel of about 10 ppm (TOC indicates the amount of carbon bound toorganic compounds and hence serves as a measure for water quality). Therelevant details of this set of experiments and results are tabulated inTable 3.

TABLE 3 Initial Final Irradiation TOC TOC time Ex. Contaminant (ppm)(ppm) (min) 18 Phenol (50 ppm) 38 6 80 19 Chlorobenzene (50 ppm) + 70 1160 Phenol (50 ppm) 20 Chlorobenzene (50 ppm) + 87 8 60 Dimethylacetamide (100 ppm)

Examples 21-23 Water Purification: Decomposition of Organic Contaminantsin Aqueous Medium in the Presence of the Compound of the Invention UnderVisible Light Irradiation

The compounds of Examples 5, 6 and 7 were tested for their ability topurify water contaminated with organic pollutants. The experiments werecarried out in a 250 mL cylindrical-shaped glass vessel at roomtemperature under air and at a neutral pH. The tested catalyst (200 mg)was suspended in water (200 ml). The sample further contains the organiccontaminant as tabulated in Table 4 (the mixture was stirred in the darkfor about 1 hour followed by filtration and measurement of adsorbedmolecules by UV).

The sample was exposed to light irradiation at wavelength 385-740 nm.For visible light irradiation, a 422 nm cut-off filter was used. 300 WXe arc lam (Max-302, Asahi Spectra) was used as the light source. Lightintensity was 70 mW/cm² and the lamp was located at a distance of 10 cmfrom the sample. Experimental details are set out in Table 4.

TABLE 4 Pollutant Light Irradiation Example Catalyst [concentration](wavelength) 21 Bi⁰ doped- Toluene 420-740 nm BiOCl_(0.875)Br_(0.125)[470 ppm] of Example 5 22 Bi⁰ doped- MB 385-740 nmBiOCl_(0.875)Br_(0.125) [10 ppm] of Example 6 23 Bi⁰ doped-BiOBrCarbamazepine 420-740 nm of Example 7 [60 ppm]

The sample was periodically tested to determine the concentration of theremnant organic pollutant. To this end, 5 ml aliquots were periodicallytaken from the sample and centrifuged at 6000 rpm for ten minutes toremove the catalyst particles (in Examples 21 and 23, the aliquots weretaken at t=0 min, 5 min, 20 min and 30 min and in Example 22, aliquotswere taken at t=0 min, 30 min, 60 min, 90 min and 120 min). UVabsorption spectra of the pollutant in the aliquot were recorded. Inorder to illustrate the decrease of the concentration of the pollutantwith the passage of time, the ratio C_(L)/C₀ was plotted as a functionof time. FIGS. 21 to 23 show the plots generated for Examples 21 to 23,respectively. The concentration of the pollutant drops sharply in thepresence of the Bi⁰ doped-catalyst of the invention under lightirradiation.

Example 24 (Comparative) and 25 (of the Invention)

The activity of the BiOCl_(0.875)Br_(0.125) photocatalyst disclosed inExample 5 of WO 2012/066545 was compared with that of theBi⁽⁰⁾doped-BiOCl_(0.875)Br_(0.125) compound of Example 4 (supra). Thephotocatalysts were tested for their ability to reduce phenol pollutionin water, in response to light irradiation. Two separate samples wereprepared according to the experimental conditions set forth in respectto Example 18 (Example 25 corresponds to Example 18). The TOC values ofthe two samples were measured periodically and the results are tabulatedbelow, showing the reduction in the TOC of the aqueous solution with thepassage of time.

TABLE 5 Example 24 (comparative) Example 25 (of theBiOCl_(0.875)Br_(0.125) of invention) Bi⁽⁰⁾ doped- WO 2012/066545BiOCl_(0.875)Br_(0.125) Time* TOC TOC (min) (ppm) (ppm) 0 38 38 10 36 3120 35 23 40 33 19 60 30 11 80 29 6 *Time elapsed from the beginning ofirradiation

The results shown in Table 5 demonstrate that the presence of bismuthmetal dopant in the catalyst accounts for stronger photocatalyticactivity, allowing rapid and effective destruction of phenol pollutantin water.

Example 26 Thin Film Formulation

Bi³%-Containing Adhesive Solution

Tetraethyl orthosilicate (TEOS; 5.2 gram), de-ionized water (2.7 gram)and ethanol (6 gram) were mixed together in the presence of nitric acid(pH=2) at 60° C. for 20 minutes. Pluronic P123 (0.15 gram) and polyvinyl alcohol (0.18 gram), both dissolved in 4 gram of ethanol were thenadded and the stirring continued for an additional hour at 60° C. toform the “glue” solution siloxane. Bismuth nitrate (0.0066 mole) is thenadded to the siloxane solution and the resultant mixture is vigorouslymixed using homogenizer to form homogeneous blend.

Halide-Containing Solution

In case of BiOCl Film: CTAC aqueous solution (8.53 g of 25 wt %solution) is placed in a spraying device.

In case of BiOBr Film: CTAB ethanol solution (2.43 g of CTAB dissolvedin 12 ml of EtOH) is placed in a spraying device.

Coating Procedure

Microscope glass slides were carefully cleaned using acid piranha (a 3:1mixture of sulfuric acid and hydrogen peroxide). Then, the slide isimmersed in the Bi³%-containing solution and dip coated, followed byspraying the halide-containing solution onto the coated glass slide. Inorder to achieve a full removal of the organic residues from the finalfilm, and for a better adhesion of the siloxane matrix, a calcinationsstep is applied. To this end, the slide is placed in an oven at roomtemperature. The temperature of the oven is increased gradually at arate of 3 degrees per minute up to a temperature of 400° C. The slide isheld in the oven at 400° C. for four hours and is then cooled down toroom temperature. A thin uniform film is obtained, in which the catalystis affixed onto the surface of the glass slide.

The invention claimed is:
 1. A process for the preparation of bismuthoxyhalide, comprising combining at least one bismuth salt and at leastone halide source in an acidic aqueous medium in the presence of areducing agent, and isolating a precipitate formed, wherein theso-formed bismuth oxyhalide is doped with elemental bismuth Bi⁽⁰⁾,wherein the so-formed bismuth oxyhalide is doped with elemental bismuthBi⁽⁰⁾ at a doping level of not more than 7 molar %, relative to thetotal amount of the bismuth.
 2. A process according to claim 1, whereinthe halide source is an organic halide salt.
 3. A process according toclaim 2, wherein the organic halide salt is selected from the groupconsisting of quaternary ammonium salts represented by the formulasN⁺R₁R₂R₃R₄C⁻, N⁺R₁R₂R₃R₄Br⁻ and their mixture, wherein R₁, R₂, R₃ and R₄are alkyl groups, which may be the same or different.
 4. A processaccording to claim 1, wherein the reducing agent comprises hydride.
 5. Aprocess according to claim 4, wherein the reducing agent is borohydride.6. A process according to claim 1, wherein the acidic aqueous mediumcomprises an organic acid.
 7. A process according to claim 1, whereinthe bismuth oxyhalide is selected from the group consisting of BiOCl,BiOBr and BiOCl_(y)Br_(1-y) wherein y is in the range from 0.5 to 0.95.8. A process according to claim 7, wherein the bismuth oxyhalide isBiOCl_(y)Br_(1-y) wherein y is in the range from 0.6 to 0.95.
 9. Aprocess according to claim 1, wherein the bismuth oxyhalide is selectedfrom the group consisting of: Bi⁽⁰⁾doped-BiOCl; Bi⁽⁰⁾doped-BiOBr; andBi⁽⁰⁾doped-BiOCl_(y)Br_(1-y) wherein y is in the range from 0.5 to 0.95.10. A process according to claim 1, wherein the doping level of Bi(0) isfrom 0.1 to 3 molar %.
 11. A process for the preparation of bismuthoxyhalide, comprising combining at least one bismuth salt and at leastone halide source in an acidic aqueous medium in the presence of areducing agent, and isolating a precipitate formed, wherein theso-formed bismuth oxyhalide is doped with elemental bismuth Bi⁽⁰⁾,wherein the halide source is an organic halide salt.
 12. A process forthe preparation of bismuth oxyhalide, comprising combining at least onebismuth salt and at least one halide source in an acidic aqueous mediumin the presence of a reducing agent, and isolating a precipitate formed,wherein the so-formed bismuth oxyhalide is doped with elemental bismuthBi⁽⁰⁾, wherein the reducing agent comprises hydride.